Modern large scale excavators are capable of moving enormous amounts of material during each load cycle. For example, it is common for excavators to move loads weighing in excess of two hundred tons. Large scale excavators are often used in the mining industry, but also find application in other earth moving applications, such as sand pits.
There are several types of large scale excavators. For example, one common large scale excavator is the so-called dragline excavator. Another example is the shovel type excavator. Those skilled in the art will be familiar with the general operation of these and similar excavators.
Efficiency and productivity are extremely important in large scale excavation projects. Efficiency is often determined by the weight of the material moved per dig cycle. Of course, the actual weight of material loaded during any given dig cycle varies. Hence, it is important to know the weight of the material moved during each dig cycle.
Likewise, concerns for equipment and safety make it equally important to know the weight of the material loaded. For obvious reasons, it is undesirable to exceed the load weight limitation of an excavator. Similarly, loss of productivity due to equipment problems may prove extremely costly.
In response to these concerns for productivity, efficiency, and equipment operability, several systems and methods have been developed to either directly or indirectly measure the weight of a suspended load in an excavator bucket. Indirect systems monitor parameters such as motor power, rope length and bucket geometry to estimate the weight of the load. Examples of prior art systems include DIGIMATE.RTM./Plus production monitoring system and the BOOMSETNRY.RTM./Plus antitightline system, both of which are manufactured and sold by General Electric, the assignee of the current application.
Common to many indirect load measuring systems are means for determining the three dimensional position of the suspended load. With dragline and shovel type excavators, the position in space of a suspended load is determined by three independent operator controlled motion drives. First, a hoist drive raises or lowers the suspended load. Second, a drag/crowd drive moves the suspended load in or out (relative horizontal movement). Finally, a swing drive rotates the structure and the suspended load from side to side about a centerline of swing or swing axis.
Those skilled in the art will recognize that while the hoist and drag/crowd motion drives are independently controlled, the motion of one drive affects the load reflected to the other motion drive due to the geometrical relationship of the structure, the ropes and the suspended load. Moreover, when the swing drive rotates the excavator, the suspended load should be constrained to an essentially circular path. This is accomplished by the hoist and drag/crowd motion drives. FIGS. 1 and 2, discussed in more detail below, provide examples of typical dragline and shovel type excavators and may be helpful in understanding the geometric relationships between the hoist, drag/crowd and swing motion drives.
There is no economically practical method to directly measure the pull in the ropes on a large mining excavator. Earlier indirect load systems measured hoist and drag motion drive DC motor armature currents and attempted to calculate motor output torque with sufficient accuracy. The earlier attempts with drives that operated at constant motor field strengths when lifting a suspended load employed torque per armature ampere "constants" to calculate torque. In reality, these "constants" were not quite constant. Further, these earlier methods prove inadequate for use with more modern constant horsepower hoist and drag/crowd drives. These modern drives operate at variable motor field strengths. Subsequently, methods emerged to calculate the torque per armature ampere as a function of motor field strength (i.e., motor magnetic flux).
In prior art indirect load measuring systems, once hoist and drag motor torques are determined, the system must properly account for the torques required to accelerate the suspended load. Prior art methods limited armature current observations to time periods when hoist and drag/crowd drive speeds were nearly constant and ignored acceleration torques. Unfortunately, even with constant rope speeds, the suspended load still changes direction as it moves to the boom point thereby requiring accelerating rope pulls. Additionally, hoist and drag rope pulls are required to constrain the suspended load to a circular path when the excavator rotates or swings. These centripetal rope pulls are a function of swing speed and suspended load weight. Moreover, prior art indirect load measuring systems typically ignore these centripetal rope pulls.
One overwhelming challenge of any scheme utilizing hoist and drag/crowd drive torques is in accurately allocating to each drive the acceleration torques that exist for both the drive machinery and the suspended load. The system must then geometrically resolve the remaining static pull torques for each drive into vertical rope pull components equal to the weight of the suspended load.
There is a need for an indirect suspended load measuring system that eliminates the difficulty in properly allocating acceleration and static load torques between the hoist and drag/crowd drives. There is also a need for an indirect load measuring system that requires motor torque calculations for only a swing drive motor. There is further a need for a system having an accuracy that exceeds that of the prior art.